The present invention relates in general to heat exchangers and in particular to a new and useful modular arrangement for the shipment and assembly of heat exchanger units.
In recent years, Heat Recovery Steam Generators (HRSG's) have usually consisted of a vertically oriented heat exchanger comprising spirally-finned tubes located inside an externally supported box type structure. High temperature turbine exhaust gas passes through the box and over the tubes in order to recover the heat from the gas.
For economic reasons, it has been common practice to incorporate a modular type design which utilizes a shop fabricated and shop assembled heating surface arrangement. Larger HRSG's are generally two or more modules wide.
Known modular arrangements typically comprise a finned tube heating surface that is bundled complete with top and bottom headers. For ease of handling, these modular shop assembled packages are assembled with an integral shipping truss assembly built of commercially available structural steel shapes.
The finned tube heating surface is shipped in a horizontal position and rotated at the erection site to a vertical orientation. When in its final position, support for the heating surface is normally provided by base frame steel which comprises a part of the shipping truss assembly.
The known box type structure designs comprise internally insulated and lined casing panels which incorporate a cold casing design. These panels can be either a part of the shipping module or they can be installed after the HRSG pressure part modules have been placed in their final position. Column steel attached to these panels provides the overall strength and stability for the total HRSG structure by providing side to side as well as fore and aft restraint against potential loadings which could occur as a result of wind and seismic conditions.
After the modules are rotated into a vertical position and set side by side, the temporary shipping frames between the modules are removed and the pressure parts are left free-standing. Where the shipping steel has been removed, spaces or gas lanes are formed between adjacent modules and between the sidewall casing and the pressure parts. In order to maintain the boiler operating efficiency, it is necessary to block these lanes in order to force the gas through the spirally finned tubes of the heat exchanger by using plates which function as gas barriers.
Support attachments for these gas barriers on most known designs are limited to welds between the finned surface of the heat exchanger and the barrier plates. In addition, intermediate horizontal tie steel, which is used to restrain and support the module load during shipment and to prevent buckling and excessive vibrations during boiler operation, is often directly attached to the spirally finned surface by attachment welds.
However, a major problem exists in that by supporting loads through weld attachments made to the finned surface of the heat exchanger, there is not a high degree of reliability. Because the fins are typically thin (0.06 inches), it is difficult to obtain an effective strength weld attachment. Because the spirally finned tubes are either water or steam cooled, they expand thermally at a different rate than the barrier plates or tie steel which are exposed to the hot turbine exhaust gas. Thus, the rigid welded connections between the pressure parts and the tie or barrier steel are found to result in reduced flexibility at the weld attachment to the fin. This reduced flexibility has often led to attachment failure.